Modal verbs can could

Use an English modal verb + infinitive form of the verb without to. Do not change their form:

She could speak German and Spanish.(NOT She could to speak...)

Could you speak German before you went to live in Germany? (NOT Did you could speak German...)

1. Ability

Use the following English modal verbs to express ability:

Present tense: can, can’t, be able to, manage to

I can’t drive.

I manage to concentrate, but it’s difficult sometimes.

I’m able to speak three languages.

Past tense: could, couldn’t, be able to, manage to

They weren’t able to find the address of the shop.

Perfect tenses: be able to, manage to

Have you managed to complete the assignment yet?

Future tense: be able to, manage to

He won’t be able to come to the meeting this afternoon.

Use can/could to describe general ability in the present and past. This is more common than using be able to:

Can she remember anything about incident? = (Is she able to remember?)

She could speak German and Spanish but she couldn’t remember what happened when she was attacked.

To express ability related to one specific occasion in the past use couldn’t, was able to, wasn’t able to, but NOT could

The police were able to determine that she could speak German and Spanish. (not The police could determine that she...)

She couldn’t remember what had happened.

Sometimes, we use manage to to demonstrate that something was hard/difficult to do/achieve:

I’ve finally managed to work out what the problem is!

For perfect or future forms, use be able to or manage to:

Apparently the police have been able to identify her.

The police have managed to locate her family and inform them of the attack.

Soon she’ll probably be able to remember more about the incident. (NOT Soon she can probably remember more...)

2. Other ways to use can

Use can as an alternative to mean sometimes:

People can do strange things when they’ve experienced such a shock. (= people sometimes do strange things/ have strange behaviour.)

Use can for asking and giving permission:

Can I borrow your laptop this morning?

Yes, you can borrow it, but I need it back this afternoon.

3. Possibility
Use must, might, could, couldn’t and can’t when there exists evidence, information or you believe that something is probably or possibly true (or untrue). Choose the appropriate English modal verb depending on the strength of the evidence which supports the ideas.

Highly unlikely: can’t, couldn’t

Highly likely: must

Possible: might, may, could, may not, might not

Could, may and might communicate the same degree of possibility:

She may/might/could remember some details about the incident.

Couldn’t and can’t express the same degree of probability. Normally they are used to talk about the past:

The authorities realised she couldn’t be American. (= it was very unlikely that she was American)

May not and might not do are not the same as couldn’t when expressing probability:

The shopping mall may/might not be open tomorrow because of the public holiday. (NOT the shopping mall couldn’t be open...)